This invention relates generally to a solder stencil for applying a solder paste to a pad in the manufacture of integrated circuits. More particularly, the present invention is directed to reducing the creation of solder balls.
Solder stencils are generally utilized in the manufacture of contacts on printed circuit boards to define the shape of a solder paste on the surface to be soldered. Solder paste is a suspension of flux and small spheres of solder particles. When heated, the paste forms a solder which secures two metal pieces together. Solder paste is screen printed onto a circuit board using a stencil. The stencil is a thin sheet of metal that has a desired hole pattern formed in the stencil.
In a reflowing process, the solder melts, forming a mechanical and electrical connection between printed circuit board land pads and an electrical component. Flux encourages the formation of this connection. Flux may be removed through a cleaning process or left on the board with the so-called no clean process.
One problem which may occur with solder paste is the formation of solder balls. Solder balls are small spherical solder particles which are formed away from the main solder pool during reflow. If these balls become loose at any time after the product is manufactured, they can roll to a position where they can short an electrical component. Solder balling may occur around components and over the board. Large solder balls may be associated with small and low clearance passive components such as 0805 and 1206 resistor and capacitor connection sites. The large solder balls may not be removed in the cleaning process and will persist with the no-clean process.
In addition, excessive solder balling may prevent the solder from making a good solder joint fillet. Thus, the formation of solder balls may create reliability issues. There has been a continuous effort to minimize the occurrence of solder balling. See C. A. Harper, Electronic Packaging and Interconnection Handbook, 2d Edition, McGraw Hill (1997), at p. 5.54.
It would be highly desirable to have a technique for forming soldered connections using solder paste which reduces the occurrence of solder balling.
In accordance with one aspect of the present invention, a solder stencil includes a body having at least one solder aperture. The aperture has a concave edge profile.
In accordance with another aspect of the present invention, a solder stencil for depositing solder paste on a pad includes a plate having at least one solder aperture. The aperture has an area less than the area of the pad.
In accordance with still another aspect of the present invention, a method of depositing solder paste includes the step of forming a deposit of solder paste on the pad. The deposit has an area of less than the area of the pad.
In accordance with another aspect of the present invention, a pad for forming a soldered contact includes a pad and a layer of solder paste on the pad. The solder paste has a concave edge profile.
In accordance with yet another aspect of the present invention, a pad for forming a soldered contact includes a pad and a layer of solder paste on the pad. The layer leaves a concave portion of the pad uncovered by solder paste.
In accordance with but another aspect of the present invention, a method of depositing solder paste includes the step of forming a deposit of solder paste with a concave edge. The solder paste is then deposited on the pad.
In accordance with another aspect of the present invention, a method of depositing solder paste includes the step of forming a solder paste deposit. The solder paste is deposited on the pad such that the area of the solder paste is less than the area of the pad.
These aspects of the present invention may be advantageous for a number of reasons including reducing the occurrence of solder balls.